Wire troubleshooting is still very much a hands-on art that has changed little over the last 40 years. Among the techniques in current use are visual inspection, impedance testing, and reflectometry.

Visual inspection is still the most common way to check for wiring failures. It entails accessing the cables and then carefully checking the insulation for holes and cracks, often no larger than the head of a pin. Whole sections of wiring may never get inspected: chafed insulation can be hidden under clamps or around corners, or within multiwire bundles, each consisting of 75 or more wires. For example, many wire bundles are built right into the walls of aircraft.

Another approach involves measuring the cable's resistance and/or capacitance. A low resistance means the cable is "good," and a high resistance means that it is broken. Capacitance is proportional to cable length. While these methods can locate a hard fault on a single (unbranched) cable, they cannot locate small faults or faults on branched networks. To find small faults such as those left after an arc fault, a very high voltage (500 V or more) can be placed between adjacent, supposedly unconnected wires. Current leakage from one wire to another can indicate degraded or damaged insulation, although it cannot locate it. To actually locate a small fault, inert gas (such as helium) can be injected near the wire, decreasing the breakdown voltage and causing a tiny arc where the wire insulation is compromised, thus locating small faults with moderate voltage levels. This method is limited by physical access to all parts of the wire under test.

Time domain reflectometry (TDR) is customarily used to trace wiring problems. A short, typically rectangular pulse is sent down the cable, and the cable impedance, termination, and length give a unique temporal signature to the reflected signal. A trained technician then interprets the signature to determine the health of the cable. Such signal interpretation is particularly necessary for aircraft systems, where wires branch into complicated network structures and connect to active avionics. The running joke about TDR is that it requires a Ph.D to use. There are other flavors of Reflectometry, too. Standing-wave (SWR) and frequency-domain reflectometry (FDR) involve sending a set of stepped sine waves down the wire and measuring the magnitude and / or phase of the reflected wave.

Spread Spectrum Reflectometry

Today’s reflectometry methods are not able to locate the tiny faults left after an arc fault event, because their impedance discontinuity is too small to create a measurable reflection. On the other hand, if the fault could be found during the few milliseconds the arc occurs, it would be an actual short circuit, which returns plenty of reflected power! This is the concept that led to the development of Spread Spectrum Time Domain Reflectometry (SSTDR) at LiveWire Test Labs.

Spread Spectrum signals have been used in communication and radar for over 50 years. Direct Sequence Spread Spectrum (DSSS) communication uses a high speed pseudo noise (PN) code multiplexed with existing digital data to spread the spectrum, increase the number of simultaneous users on the line, and reduce the effects of noise and jamming. This same ability to reduce interference with other “users” and to resist “jamming” provides the ability to test live wires in flight without either interfering with the avionics signals or being corrupted by them.

The basic spread spectrum system is shown in Figure 1. In order to guarantee no interference, the PN code is very small (25-70 dB down) compared to the data signal. In fact, it is below the noise margin of the data. The PN code is added to the data/noise signal, and the combined signal is transmitted down the wire, where it reflects from the end of the wire. The combined incident/reflected signal is correlated with the PN code. This correlation is high if the two codes are synchronized, and low if they are not. This creates a set of peaks that correspond to the length of the wire and type of fault, as shown in Figure 2. Thus the system is capable of running live, with the test signal completely buried within the system noise. It can locate intermittent faults a few milliseconds long to within a few centimeters over tens to hundreds of meters of wire.

Our research in conjunction with our industrial partners is working towards integrating this technology directly into the arc fault circuit breakers, into the connectors between the wires, and eventually into the wires themselves.

Spread Spectrum System


STDR response for an 80 foot wire (paired single 22 gauge wires bundled with other wires) that is short or open circuited on the end.

For more in-depth technical information, please contact us